An alternating multi-layer film comprising layers having a low refractive index and layers having a high refractive index can be used as an optical interference film which selectively reflects or transmits light having a specific wavelength due to structural optical interference between layers. This multi-layer film can obtain as high reflectance as that of a film comprising a metal by gradually changing its film thickness or by laminating together films having different reflection peaks and can be used as a film having metallic luster or a reflection mirror. Further, when this multi-layer film is stretched only in one direction, it becomes a polarization reflection film which reflects only a specific polarization component. It is known that when it is used in a liquid crystal display, it can be used as a brightness improving film for liquid crystal displays.
In general, a phenomenon such as “increased reflection” that light having a specific wavelength is reflected is seen in a multi-layer film consisting of layers having a thickness of 0.05 to 0.5 μm and different refractive indices according to the difference in refractive index between one type of layers and the other type of layers, film thickness and the number of layers. The reflection wavelength is generally represented by the following equation.λ=2(n1×d1+n2×d2)(In the above equation, λ is a reflection wavelength (nm), n1 and n2 are the refractive indices of respective layers, and d1 and d2 are the thicknesses (nm) of the respective layers.)
For example, as shown in Patent Document 1, when a resin having a positive stress-optical coefficient is used in one type of layers, the refractive index of the layers is made birefringent by stretching the layers in a uniaxial direction so as to provide anisotropy to the layers, thereby expanding the difference in refractive index between layers in a stretching direction within the film plane and reducing the difference in refractive index between layers in a direction orthogonal to the stretching direction within the film plane. As a result, it is possible to reflect only a specific polarization component.
Making use of this principle, a reflection polarization film which reflects polarization in one direction and transmits polarization in a direction orthogonal to the above direction can be designed, and the desired birefringence of the film is represented by the following formulas.n1X>n2X, n1Y=n2Y (In the above formulas, n1X and n2X are the refractive indices in the stretching direction of respective layers, and n1Y and n2Y are the refractive indices in a direction orthogonal to the stretching direction of the respective layers.)
Patent Document 2 and Patent Document 3 disclose a multi-layer film in which polyethylene-2,6-naphthalene dicarboxylate (may be referred to as “2,6-PEN” hereinafter) is used in layers having a high refractive index and a thermoplastic elastomer or PEN comprising 30 mol % of isophthalic acid is used in layers having a low refractive index. These references teach a reflection polarization film which reflects only specific polarization by using a resin having a positive stress-optical coefficient in one type of layers and a resin having an extremely small stress-optical coefficient (extremely rare development of birefringence by stretching) in the other type of layers.
However, when 2,6-PEN is used in the layers having a high refractive index, there is produced a difference between the refractive index in a direction (Y direction) orthogonal to the stretching direction and the refractive index in the thickness direction (Z direction) of the film after stretching in the layers. Therefore, when the draw ratio is to be increased to expand the difference in refractive index between layers in the stretching direction (X direction) so as to enhance polarization performance, the difference in refractive index between layers in the Z direction is also expanded. Therefore, there arises a problem that a hue shift of transmitted light becomes larger due to the partial reflection of diagonally incident light.    (Patent Document 1) JP-A 04-268505    (Patent Document 2) JP-A 9-506837    (Patent Document 3) WO01/47711